This LARGE 2-disc clinic-sized Obdisk is designed to be
used by doctors, midwives, ultrasonographers and
antenatal educators.

It provides a means of quickly counting the weeks and days, during pregnancy. The timing of antenatal screening tests and assistance with determining fetal viability are two areas where the use of a large Obdisk is invaluable - helping professionals to reduce the risks to pregnant women and their unborn babies.

Clinical Information and Obdisk 2L Instructions

A. Using the ‘period dates’:
1. Take the FIRST DAY of the Last Menstrual Period [LMP]. 
2. Moving the top disc, line up the LMP arrow with the date of the patient's LMP.
3. You can read off the fetal gestational age in weeks, throughout pregnancy, by following the calendar around carefully. For a reading that is as accurate as possible, adjust the discs very slightly if necessary to match up the days exactly.
4. You can read off the estimated date of delivery at the triangle marked “EDD DUE DATE”. The date of this simple calculation is therefore based only on the ‘period dates’. 

B. Using the dating scan result, when available:
If a 'dating scan' is performed [using ultrasound], simply turn the top disc so that the number of weeks agrees with the date of the scan. Then read off the baby’s gestational age and due date of delivery by following the calendar around carefully. [Adjust the discs very slightly if necessary to match up the days exactly.] 

Notes:
1. The Obdisk 2L calculator is based on a pregnancy of 280 days from the LMP if there have been regular 4-week cycles, but the EDD would need a suitable adjustment for irregular cycles and a leap year. 
2. The 'period dates' and scan dates may, or may not, agree. Different hospitals have different policies about how to deal with a discrepancy between these dates, but a midwife or doctor will be able to explain this to the patient if necessary.
3. You can use the 2-disc Obdisk 2L Pregnancy Wheel either to calculate 'period dates' or scan dates - but not both at the same time. [The 3-disc Obdisk 3 model may be used for this purpose, if required.]
4. Because of the traditional way of estimating human pregnancy in the UK, a woman is considered to be 6 weeks pregnant [from the date of her LMP] when she has only been pregnant for 4 weeks [from conception]. The due date is generally estimated at 40 weeks [280 days] after the LMP, and 38 weeks [266 days] after conception.